You are given an integer number S. You can transform any integer number A to another integer number B by adding x to A. This x is an integer number which is a prime factor of A (Please note that 1 and A are not being considered as a factor of A). Now, your task is to find the minimum number of transformations required to transform S to another integer number T.
EX: 6 12
6->8-> 10->12 3次
For each test case, there will be a line with two integers, S (1<=S<=100) & T (1<=T<=1000), as described above. The last test case will be followed by a line with two 0 s denoting end of output. This case should not be processed.
每組測資都有兩的數字S (1<=S<=100) & T (1<=T<=1000)，
For every test case except the last one, print a line of the form “Case X: Y” where X is the serial number of output (starting from 1). Y is the minimum number of transformations required to transform S to T. If it is not possible to make T from S with the given rules, Y shall be -1.
對於每個一個測資，除了0 0以外 ，請應出 “Case X: Y”，X從1開始算起，Y為S轉換到T的最小次數，若是無法轉換成功請書出-1
6 12 6 13 0 0
Case 1: 2 Case 2: -1